Effect of damaged ends in papillary muscle preparations.

The mechanical characteristics of the central segment of isolated cat papillary muscle were determined with recently developed equipment. Two small sharpened stainless steel pins, inserted transversely through the muscle, were used to mark the ends of a segment not damaged by attachments. Installation of the pins did not affect the performance of the muscle. The distance between the pins was measured and controlled to produce isometric and afterloaded isotonic contractions of the segment of the muscle between the pins. Data from such contractions were compared with traditional whole muscle measurements made on the same preparation. The isometric length-tension curve of the central segment was significantly higher than that of the whole muscle, and there was no plateau of developed force at long lengths in five of six muscles studied. In the resting state, the segment was more compliant than the whole muscle for physiologic lengths and much stiffer for longer lengths. Segment velocity and shortening were significantly higher than whole muscle velocity and shortening at comparable loads.